As industrial automation advances, interconnectivity between various microprocessor based plant floor devices, such as programmable logic controllers ("PLCs"), and plant computers, becomes more and more desirable. However, as such microprocessor based devices and computers are made by various vendors utilizing their own various communication protocols, interconnectivity between such devices has been hampered, requiring application programs to be dedicated often to a single product and, consequently, non-transportable or reusable in future applications.
To help alleviate this problem, a seven-layer Open Systems Interconnection (OSI) communication model was specified by the International Standards Organization (ISO).
According to this model, layer 1 defines the physical layer, layer 2 defines the data link layer, layer 3 defines the network layer, layer 4 defines the transport layer, layer 5 defines the session layer, layer 6 defines the presentation layer and layer 7 defines the application layer. OSI specifies general guidelines, but does not specify how these guidelines are to be realized.
One protocol developed in accordance with the OSI model is commonly referred to "Manufacturing Automation Protocol", or simply MAP, which utilizes messages conforming with a protocol known as Manufacturing Message Specification, or MMS.
MAP is a communication networking standard initiated by General Motors Corporation and supported by the World Federation. It relies on the above seven independent, yet functionally supportive layers which serve as an accepted set of rules for data exchange within the manufacturing environment. The current MAP version is 3.0. MAP connectivity simplifies the task of data exchange between factory control devices and higher level manufacturing computers, typically supplied by a variety of vendors. So long as a device outputs its messages in accordance with MMS, all other devices on the MAP network should be able to recognize its messages.
Often, factory control devices, such as programmable logic controllers (PLCs) are interconnected via a local area network, or LAN, such as the SY/NET network of Square D Company, Palatine, Ill. It has been found to be beneficial to also permit these PLCs to exchange information with higher level computers.
On the PLC level, Systems Integration Specialists Co., (SISCO) of Warren, Mich., has developed an interface which couples PLCs on an Allen Bradley Data Highway local area network to a MAP network. However, in this system, the PLCs on the Data Highway network can only function as "servers", rather than as "clients" on the MAP network. Thus, they can only respond to requests from clients on the MAP network and are unable to generate their own requests out onto the MAP network. This greatly limits their overall effectiveness.
Applicant's invention is provided to solve this and other problems.